As a tire wears down its tread, the volume of the grooves therein is reduced, which can lead to poor hydroplaning performance. To increase the worn groove volume, keyhole sipes, i.e. sunken tire grooves having a keyhole cross-section, are incorporated into the tread. Each keyhole sipe typically includes a slit defining an opening at a surface of the tread and a passage formed therebelow that cooperates with the slit to increase the tread's worn groove volume.
To form these keyhole sipes, molds are equipped with sipe-forming devices, which are well known in the art of tire manufacture. The molds may be used for producing a tread member only, as for use in retreading a previously cured tire, or for forming a tread design on a green tire. Such sipe-forming devices conventionally include a base, or body member, provided with a blade that typically is made from a running length or strip of a metallic material such as steel, stainless steel, or brass. The blade is shaped to form the slit in the tire tread while the body member produces the passage underneath the tread surface, thereby producing the keyhole-shaped cross-section of the sipe. In forming the keyhole sipe, the molding device is positioned within a channel in the mold so that the blade cooperates with a surface of the channel and the body member is spaced apart therefrom, respectively, to form the slit in the tire and the passage underneath the tread surface. Next, the rubber formulation is introduced into the mold. The mold then is closed and the tread allowed time to cure. Finally, the mold is opened and the tread removed therefrom with the molding device having formed the keyhole sipe therein.
One drawback with current molding devices includes an inability of the body member to deform, or compress, during tread removal thereby placing high stress on the tread when the slit of the formed keyhole sipe is pulled around the body member of the molding device. Notably, if the body member exceeds a certain size parameter, i.e. a certain width, there is a high likelihood that the tread will be damaged, such as by being torn, during removal from the mold. Deformable body members have been proposed in an effort to replace non-deformable members to reduce the stress on the tread during removal thereof. However, one drawback with these types of current devices includes the tendency of the deformable body member to inwardly flex, or collapse, along its length under the stress of the rubber formulation when the mold is closed during the molding process. This inward distortion damages the tread during tread formation by producing a misshapen keyhole, i.e. a misshapen passage.
Because of the above limitations, current molding devices are designed according to certain size parameters to avoid damaging the tire tread during tread formation. More specifically, the body members of current molding devices typically have a width not greater than about 3.5 mm while the blade typically is not greater than about 0.8 mm in thickness. Since a larger keyhole sipe, i.e. one having a wider passage, would be better able to provide wet traction of the tire and be better able to reduce air transfer noise, the tire industry has long felt a need for a molding device having a wider body member that can produce a larger-sized keyhole sipe without causing damage to the tread during the molding process.
Accordingly, there is a need for an improved device for molding keyhole sipes larger in size, i.e. sipes having a wider passage, than currently being formed in treads that will provide a properly shaped keyhole sipe and avoid damaging the tread during formation and removal thereof from the mold, thereby providing the worn tire tread greater hydroplaning performance.